At the media manager If I click in an image I see "unique title shown" I guess that is the title tag and subtitle shown, is that the alt tag?
correct ... the unique filename is the frontend title tag for the image and the image alt text is the frontend alt property for the image
But I cheated a little bit
I adjusted my template overrides in such way that the image title is swapped with the name of the product and the alt img description is switched with the product short description.
$p2img_raw=$product->images->displayMediaThumb('class="browseProductImage" border="0" title="'.$product->product_name.'" ',true,'class="modal"');
//search the alt property in the img tag and make it a variable
preg_match_all('/alt="([^`]*?)"/', $p2img_raw, $p2alt);
//forgot what this part does ... haha
$p2alt_old = strip_tags($p2alt);
//replace the old part with the product short description
//now spit out the reworked image tag
this generates something like this (see below) without having to manually write a comment in the admin backend (just typed something here below
<img border="0" title="Snickers power bar" class="browseProductImage" alt="Caramel chocolate bar with nuts" src="/images/stories/virtuemart/product/resized/25642225.jpg">
the only thing you need to look out for, is to keep the short description within the advised limits for alt tag use. But it saves me time and google image browsing uses the adjustment i've made. But don't do this in your productdetails page, otherwise all your images will have the same name and alt text. But this gives good results for the category views.